Under the electromagnetic force compensation measurement principle, an opposing force, also referred to as compensation force, is generated which counteracts the force that is to be measured by the force-measuring device. As a result of this compensation, the movable parts of the force-measuring device which serve to receive and transfer the force are regulated to maintain a set position. Accordingly, in the case of a balance the weight of the weighing object represents the force to be measured, and the movable parts include components such as the balance pan, levers, rods, or the weighing cell.
In many cases, the compensation force is generated by an electromagnetic coil which is arranged so that it can move within an air gap of a magnet system and which carries an electric current that is sent through it. This current, referred to as compensation current, represents a measure for the compensation force that is being generated and thus represents a measure for the force that is acting on the force-measuring device. By measuring this compensation current, a corresponding measuring signal is obtained which is analyzed in a signal-processing unit and converted into results that are delivered to an indicator device.
Force-measuring devices based on electro-magnetic force compensation have the disadvantage that the power which is generated in the coil by the compensation current depends on the magnitude of the current flowing and thus on the amount of force acting at any given time. Therefore, with measurements of different forces or loads following each other, different levels of power are generated and released in the form of heat.
As a consequence of the varying amounts of heat being released and the temperature changes that occur as a result, the zero point and the span of the measurement range can become unstable. These influence factors should therefore as much as possible be kept constant, particularly in force-measuring devices which have to meet exacting requirements regarding the invariability of their measurement accuracy.
The known state of the art offers different methods of achieving a power dissipation of the coil that is constant and independent of the weighing load. It is possible to use for example an additional power-consuming device which converts additional electrical energy into heat. As a result, the total amount of power consisting of the sum of the coil power and the additionally dissipated power of the power-consuming device remains largely constant. In this arrangement, the additional power-consuming device should as much as possible be neutral in its behavior in regard to force effects, so as not to influence the compensation force being generated.
For example, in DE 28 19 451 a variety of force-neutral power-consuming devices are disclosed in the form of a power transistor, a semiconductor resistor, or a coil with bifilar windings where the two coil parts are controlled so that their currents are anti-parallel, i.e. of equal magnitude but opposite direction. In a coil with bifilar windings and equal anti-parallel current flows, two forces are produced which mutually neutralize each other, so that the result is a power-consuming device which is force-neutral in its overall effect. Depending on its implementation, this concept has the following drawbacks:                The location where the non-productive power is generated is not geometrically identical with the location where the productive power is generated. Consequently, the temperature distribution in the force-measuring device still depends on the compensation force that is to be measured.        The non-productive power is released in a coil part with anti-parallel current flows that is located within the compensation coil. With this arrangement, the degree of efficiency of the force-measuring device is reduced because only part of the coil is usable for the generation of the force even at the maximum level of compensation force.        
As a further way of implementing a force-neutral power-consuming device, the concept of adding an alternating current to the compensation current is disclosed in DE 31 36 171. Thus, the resultant average compensation force of the coil remains essentially unchanged, while an additional power consumption is achieved through the AC component of the current. By alternating the direction of the current, the differences in the two windings are averaged out. This form of heat generation has the disadvantage that extensive measures have to be taken to prevent the AC component from causing a change of the average compensation current.
This disadvantage can be circumvented by using a coil with two separate windings acting in opposite directions which are energized alternatingly. In this arrangement a current flowing through the first winding generates a first force, and a current flowing through the second winding generates a second force. As an average, the two forces will produce a resultant force which represents the effectively acting compensation force.
For the control of the two currents, a switch-over device is disclosed in CH 634 654 which serves to direct the compensation current through the first winding during a first time interval and through the second winding during a subsequent second time interval. This has the result that the total compensation current over the two time intervals remains to a large extent constant. However, the periodic switching of the current flows generates strong alternating forces in the coil and thus causes a considerable amount of audible noise.
If an effort is made to mitigate the negative effects of the abrupt periodic alternations of the current by means of filter elements or smoothing capacitors, the power dissipation in the coil will again strongly depend on the compensation force generated at any particular moment and thus on the weighing load that is present at any given time.
It is therefore an object to propose a force-measuring device which is based on the principle of electromagnetic force compensation and which is distinguished by the fact that a simple, cost-effective design and operation of the force-measuring device can be achieved while the force-measuring device at the same time meets stringent requirements in regard to its measurement accuracy and efficiency.
A further object is to propose a force-measuring device which meets the aforementioned requirements and which can be operated without making an irritating noise.
A further aim is to achieve an optimal utilization of the space available for the windings. This is the space which is allocated to the part of the coil system that carries the windings. It is in essence delimited by the dimensions of the permanent magnet, in particular the magnet gap in which the coil system is arranged.